Insulin is a 51-amino acid polypeptide composed of A and B chains connected through the C-peptide. Proinsulin, which has very little biological activity, is cleaved by proteases within its cell of origin into the insulin molecule and the C-terminal basic residue. Insulin enhances membrane transport of glucose, amino acids, and certain ions. It also promotes glycogen storage, formation of triglycerides, and synthesis of proteins and nucleic acids. The main storage site for insulin is the pancreatic islets. Antibodies to insulin are important as β-cell of islets of Langerhans and tumor (insulinoma) marker.